In recent years, it is widely carried out that a specific immunologically active substance such as a particular protein, etc. appearing in living body fluid in relation to the conditions of diseases is determined by utilizing an antigen-antibody reaction, and a result obtained is utilized for diagnosis. Various methods have been developed for biochemical measurements utilizing such antigen-antibody reaction. Illustrative of the methods are, for example, radioimmunoassay (RIA), enzyme immunoassay (EIA), enzyme-labelled immunosolvent assay (ELISA), light scattering photometry and nephelometry. Among these, RIA, EIA and ELISA need in any of the cases separation of the reaction product after the reaction with a liquid to be examined. Accordingly, these methods generally require a plenty of time and much labor for the measurement, but are now widely utilized for the reason that these methods are excellent in the quantitative results of determination.
Very recently, a photometric determination method such as a light scattering method, nephelometric method or the like capable of measuring optical changes in a test liquid occurring as a result of an immune reaction (an antigen-antibody reaction) has attracted public attention as a method for the determination of immunologically active substances. These methods are based on the principle that since change in turbidity of a liquid takes place more or less before and after the reaction between an antigen and an antibody in the liquid in compliance with the degree of reaction, determination of an immunologically active substance aimed at can be made by measuring the change with any proper photometric means. Accordingly, the light scattering method and nephelometry are distinguished by their own easiness and convenience in operation for the measurement as compared with RIA, EIA and ELISA.
On the measurement of immunologically active substances utilizing such antigen-antibody reaction, the use of particular additive has also been investigated for promoting the antigen-antibody reaction. In Japanese Patent Publn. No. Sho. 60-4938 for example, there is disclosed the use of a non-ionic surfactant consisting of polyethylene glycol and a block copolycondensate having the structural formula: HO(CH.sub.2 CH.sub.2 O).sub.a (CH.sub.3 CHCH.sub.2 O).sub.b (CH.sub.2 CH.sub.2 O).sub.c H and the like. In Japanese Laid-open Patent Appln. No. Sho. 59-43362, there is disclosed the use of a compound of the general formula: ##STR1## as such additive. Further, Japanese Laid-open Patent Appln. No. Sho. 58-47256 discloses that polyethylene glycol can be used as the additive in the system where an antigen or antibody is carried on insoluble fine particles and is subjected to an antigen-antibody reaction in a solution.
However, a satisfactory result was not always obtained in case of using such known additive. For example, in case of the use of such known additive for a photometric measurement, e.g. nephelometry or turbidimetry, of a reaction mixture obtained by reacting an antigen with an antibody in a solution, substances which are coexistent in the solution other than the object of measurement, such as fat and oil or protein, permit the occurrence of turbidity, the so-called non-specific reaction, which may cause error in the measurement. Further, the so-called prezone phenomenon may take place wherein a result of the measurement rather indicates a low concentration of a substance to be measured notwithstanding it actually exists at a high concentration, or alternatively, a result of the measurement may be inaccurate at a very low concentration. The additives may cause a non-specific reaction or the prezone phenomenon also in case of EIA or RIA. Thus, the use of such additive rather involves troublesome problems though it promotes the reaction to a certain degree.
Under the circumstances above mentioned, there is a great demand for developing a new method for measuring an immunologically active substance in high accuracy by utilizing an antigen-antibody reaction in a liquid, wherein all of the drawbacks as seen in the prior art methods are overcome and for providing a new reagent utilizable for such new method.